A reluctance type motor comprising a stator having a plurality of magnetic poles and a rotor coaxially and rotatably disposed with respect to the stator and having a plurality of salient poles, is known so that the rotor can be rotated by magnetic attraction force interacting between successively excited stator magnetic poles and their associated rotor salient poles.
The reluctance type motor is advantageous in that its output torque is relatively large and it does not require a magnet rotor. On the other hand, a conventional reluctance type motor has a disadvantage in that its application field is limited due to a difficulty in operating at a high speed.
That is, an exciting coil of the reluctance type motor has a large inductance and, therefore, a magnetic energy stored in the exciting coil becomes very large. Accordingly, it requires a considerably long time for storing or discharging the magnetic energy. In other words, a building-up and a trailing-edge of the exciting current are undesirably delayed.
Furthermore, since the rotor has many salient poles, a storage and discharge of the magnetic energy in a magnetic pole are numerously repeated during one complete revolution of the rotor. Therefore, a torque reduction is generated due to the delay in its building-up of the exciting current, and a counter torque is also generated due to the delay in its trailing-edge of the exciting current.
As a result, an efficiency of the motor is lowered and its rotational speed is markedly reduced. Even if the numbers of the disposed salient poles and magnetic poles are increased to enlarge the output torque of the motor, the stored magnetic energy is further increased. This results in the increase in the building-up time and the trailing-edge time of the exciting current, thereby largely decreasing the rotational speed. Thus, the motor is not only required to become complicated in its constitution but also required to becomes large in its size.
Especially, in the case of a three-phase full-wave reluctance motor requiring numerous salient poles and magnetic poles, the numbers of the salient poles and magnetic poles have to be increased by more than twice, respectively (compared with other types of motors), thereby making a high-speed driving and a reduction of size difficult.
Moreover, if a high-voltage electric power source is used in order to make steep the building-up of the current supplied to the exciting coil, the exciting current steeply builds up and generates vibrations and electric noises when the magnetic pole has reached its magnetic saturation point.
Still further, where an electric power voltage has to be raised to increase the output torque, an electric power voltage of more than 1000 volts is required to obtain a generally utilized rotational speed (i.e., a several thousands rpm). Thus, the motor loses its practical merit.